I have just restarted the GPU folding on my gf's rig and I must say the whole computer gets quite laggy, its an i3 530, with a GT 240 ddr5 and 4gb RAM.

Based on my experience with my own rig (in sig), this is not normal, I could game while folding, here I have a hard time using FF. On average the CPU usage is at around 20% on 2 threads, peaking at 100% whenever I start to do a little something. The other 2 threads basicly peak at 20% max. The priority is set to low.